Ultrafine composite fiber, ultrafine fiber, method for manufacturing same, and fiber structure

ABSTRACT

An ultrafine composite fiber of the present invention is obtained by heating and melting a composite-resin-formed product in front of a supply-side electrode and/or in a space between electrodes and extending the composite-resin-formed product by electrospinning, wherein the composite-resin-formed product is a solid-state composite-resin-formed product having two or more phases and including a resin that has a volume specific resistance of 10 15  Ω·cm or less, and that is exposed on 30% or more of a surface of the composite-resin-formed product. With this, an ultrafine composite synthetic fiber and an ultrafine synthetic fiber can be obtained by electrospinning, without a solvent being mixed in a supply resin, and further, a method for manufacturing an ultrafine composite fiber, as well as a fiber structure containing an ultrafine composite fiber, are provided.

TECHNICAL FIELD

The present invention relates to an ultrafine fiber formed usingelectrospinning, a method for manufacturing the same, and a fiberstructure containing an ultrafine composite fiber obtained by theaforementioned method.

BACKGROUND ART

Conventionally, synthetic fibers such as polyethylene terephthalate(PET) fibers, nylon fibers, and polyolefin fibers have been manufacturedby melt-spinning in general. With the melt-spinning, however, it isdifficult to obtain ultrafine fibers, and particularly it is difficultto obtain fibers having a diameter of not more than 8 μm (single fiberfineness: about 1 decitex) stably.

On the other hand, the following patent documents 1 to 3 proposeelectrospinning methods as methods for obtaining ultrafine fibers.Solid-melt-electrospinning is disclosed in Patent Document 1.

Patent Document 1: JP 2007-239114 A

Patent Document 2: JP 2007-197859 A

Patent Document 3: JP 2005-154927 A

The conventional electrospinning methods, however, have problems asfollows. A polymer that cannot be electrified easily is not formed intofine fibers easily even if a voltage is applied thereto, and it isdifficult to apply such a polymer to electrospinning. Further, in thetechniques disclosed in Patent Documents 1 to 3, a supply resin beforespinning has to be dissolved or dispersed in a solvent, which could leadto a problem of components originating from the solvent remaining in anobtained fiber. If a solvent remains in an obtained fiber, componentsoriginating from the solvent bleed out therefrom later, which couldcause various problems.

DISCLOSURE OF INVENTION

In order to solve the aforementioned problems in the conventionaltechniques, the present invention obtains an ultrafine compositesynthetic fiber and an ultrafine synthetic fiber by electrospinning, andprovides a method for manufacturing an ultrafine composite fiber, and afiber structure containing an ultrafine composite fiber.

An ultrafine composite fiber of the present invention is obtained byheating and melting a product formed of a composite resin (hereinafterthis product is referred to as “composite-resin-formed product”) infront of a supply-side electrode and/or in a space between thesupply-side electrode and a collection-side electrode and extending thecomposite-resin-formed product by electrospinning,

wherein the composite-resin-formed product is a solid-statecomposite-resin-formed product having two or more phases and including aresin that has a volume specific resistance of 10¹⁵ Ω·cm or less andthat is exposed on 30% or more of a surface of thecomposite-resin-formed product, and

fiber components composing the ultrafine composite fiber are in a phaseseparation state.

An ultrafine fiber of the present invention is obtained by removing anyof components composing the ultrafine composite fiber.

A method for manufacturing an ultrafine composite fiber according to thepresent invention includes the steps of

supplying a composite-resin-formed product to a supply-side electrode,wherein the composite-resin-formed product is a solid-statecomposite-resin-formed product having two or more phases and including aresin that has a volume specific resistance of 10¹⁵ ∩·cm or less andthat is exposed on 30% or more of a surface of thecomposite-resin-formed product;

heating and melting the composite-resin-formed product in front of thesupply-side electrode and/or in a space between the supply-sideelectrode and a collection-side electrode; and

extending the molten composite-resin-formed product by electrospinning.

A fiber structure of the present invention is obtained by heating andmelting a composite-resin-formed product in front of a supply-sideelectrode and/or in a space between the supply-side electrode and acollection-side electrode and extending the composite-resin-formedproduct by electrospinning,

wherein the composite-resin-formed product is a solid-statecomposite-resin-formed product having two or more phases and including aresin that has a volume specific resistance of 10¹⁵ Ω·cm or less andthat is exposed on 30% or more of a surface of thecomposite-resin-formed product, and

fiber components composing the ultrafine composite fiber are in a phaseseparation state.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic explanatory view of an electrospinning deviceaccording to one example of the present invention.

FIG. 2 is a schematic explanatory view of an electrospinning deviceaccording to another example of the present invention.

FIG. 3 is a scanning electron photomicrograph (SEM, magnification:20,000 times) of a cross section of an ultrafine fiber obtained inExperiment No. 3 of the present invention.

FIG. 4 is a schematic cross-sectional view of the fiber shown FIG. 3.

FIG. 5 is a scanning electron photomicrograph (SEM, magnification:10,000 times) of a cross section of an ultrafine fiber obtained inExperiment No. 5 of the present invention.

FIG. 6 is a schematic cross-sectional view of the fiber shown in FIG. 5.

FIG. 7 is a scanning electron photomicrograph (SEM, magnification: 4,000times) of a cross section of an ultrafine fiber obtained in ExperimentNo. 6 of the present invention.

FIG. 8 is a schematic cross-sectional view of the fiber shown in FIG. 7.

FIG. 9 is a scanning electron photomicrograph (SEM, magnification:10,000 times) of a cross section of an ultrafine fiber obtained inExperiment No. 10 of the present invention.

FIG. 10 is a schematic cross-sectional view of the fiber shown in FIG.9.

FIG. 11 is a schematic explanatory view of an electrospinning deviceaccording to another example of the present invention, to performelectrospinning with respect to a plurality of composite fibers as a rawmaterial.

FIG. 12A shows an exemplary cross section of a composite fiber as a rawmaterial, which is a composite-resin-formed product, used in Example 2of the present invention. FIG. 12B shows an exemplary cross section ofan ultrafine composite fiber obtained in Example 2 of the presentinvention.

FIG. 13A shows an exemplary cross section of a composite fiber as a rawmaterial, which is a composite-resin-formed product, used in Example 3of the present invention. FIG. 13B shows an exemplary cross section ofan ultrafine composite fiber obtained in Example 3 of the presentinvention.

FIG. 14 is a scanning electron photomicrograph (SEM, magnification:2,000 times) of an ultrafine fiber obtained in Example 2, Experiment No.13 of the present invention, in a state before EVOH was removed.

FIG. 15 is a scanning electron photomicrograph (SEM, magnification:2,000 times) of the ultrafine fiber obtained in Example 2, ExperimentNo. 13 of the present invention, in a state after EVOH was removed.

FIG. 16 is a scanning electron photomicrograph (SEM, magnification:2,000 times) of an ultrafine fiber obtained in Example 2, Experiment No.14 of the present invention, in a state after EVOH was removed.

FIG. 17 is a scanning electron photomicrograph (SEM, magnification:1,000 times) of an ultrafine fiber obtained in Example 2, Experiment No.15 of the present invention, in a state after EVOH was removed.

FIG. 18 is a scanning electron photomicrograph (SEM, magnification:5,000 times) of an ultrafine fiber obtained in Example 3, Experiment No.18 of the present invention, in a state before EVOH was removed.

FIG. 19 is a scanning electron photomicrograph (SEM, magnification:5,000 times) of an ultrafine fiber obtained in Example 3, Experiment No.18 of the present invention, in a state after EVOH was removed.

FIG. 20 is a scanning electron photomicrograph (SEM, magnification:5,000 times) of an ultrafine fiber obtained in Example 3, Experiment No.19 of the present invention, in a state after EVOH was removed.

FIG. 21 is a scanning electron photomicrograph (SEM, magnification:5,000 times) of an ultrafine fiber obtained in Example 3, Experiment No.20 of the present invention, in a state after EVOH was removed.

DESCRIPTION OF THE INVENTION

In the present invention, a composite-resin-formed product in a solidstate having two or more phases is used as a raw material, one or moreresins in the composite resin have a volume specific resistance of 10¹⁵Ω·cm or less, and this resin component is exposed on 30% or more of asurface of the composite-resin-formed product. Thiscomposite-resin-formed product is electrified when passed through asupply-side electrode in a space between electrodes, and is extended byelectrospinning, whereby an ultrafine composite fiber and an ultrafinefiber, which were difficult to obtain by a conventional technique, canbe obtained without use of an organic solvent. In other words, anorganic solvent is not mixed in a supply resin before spinning, and theresin alone can be subjected to spinning. Accordingly, the presentinvention has an advantage in that no solvent is contained in anobtained fiber.

The inventors of the present invention examined why specific resinscannot be extended or drawn efficiently in solid-melt-electrospinning.Consequently, they found that some of specific resins such aspolypropylene (PP) have high volume specific resistances of 10¹⁶ to 10²⁰Ω·cm, and such resins are not easily electrified even when a voltage isapplied thereto, and it is difficult to form them into ultrafine fibers.Typical resins have volume specific resistance as shown below.

TABLE 1 Volume specific resistance Resin (Ω · cm) Polyethylene 10¹⁶~10²⁰Polypropylene 10¹⁶~10²⁰ Polylactic acid 10¹⁶~10¹⁷ Polyurethane 10¹³~10¹⁵Nylon 10¹³~10¹⁴ Polyester 10¹²~10¹⁴ EVOH 10⁷~10⁹ Note 1: EVOH is anabbreviation for an ethylene-vinyl alcohol copolymer Note 2: Data shownin Table 1 are based on “Plastic Data Book”, edited by Asahi KaseiAmidas Corporation, Plastics Editorial Section, published on Dec. 1,1999 by Kogyo Chosakai Publishing Inc., page 186.

Then, the inventors considered forming resin components having lowvolume specific resistance into a composite-resin-formed product withoutusing a solvent. Various complex methods for composite-resin-formedproducts are available, and exemplary types of fiber-likecomposite-resin-formed products include the splittable type, theside-by-side type, the sea-island type, the core-sheath type (thisfurther is classified into various types depending on componentsselected for the core and the sheath) from the viewpoint of thecross-sectional shape. As a result of various studies, they found that acomposite-resin-formed product that contains a resin component having avolume specific resistance of 10¹⁵ Ω·cm or less as a raw material,wherein the resin component was exposed on 30% or more of a surface ofthe composite-resin-formed product, exhibited the greatest extensibility

In the present invention, the volume specific resistance is measuredaccording to ASTM D-257.

In the solid-melt-electrospinning, a resin electrified when the resin ispassed through a supply-side electrode is extended toward acollection-side electrode at a high speed by an electric attractionforce. Therefore, a resin having a volume specific resistance exceeding10¹⁵ Ω·cm is not suitable for electrospinning since it is not easilyelectrified. In the present invention, however, such a resin having ahigh volume specific resistance, combined with another resin having avolume specific resistance of not more than 10¹⁵ Ω·cm upon extension, ismade to extend owing to the influences of the another resin. The reasonfor this is assumed as follows: when a solid composite-resin-formedproduct is heated and molten in front of a supply-side electrode, and/orin the space between the supply-side electrode and the collection-sideelectrode, a component having a volume specific resistance of not higherthan a predetermined value, exposed on a surface, at an end of a rawmaterial fiber heated and molten, is electrified enough to beelectrospun, and with this momentum, the resin having a volume specificresistance exceeding 10¹⁵ Ω·cm, which is not easily electrifiedgenerally, is extended at the same time, thereby being spun.

In the present invention, a voltage is applied across the supply-sideelectrode and the collection-side electrode in a space between theseelectrodes. The voltage preferably is 20 to 100 kV, and more preferably30 to 50 kV.

When the voltage is in the above-described range, the electrification ofresins is easily achieved, whereas sparks or corona discharge hardlyoccur between electrodes, which does not lead to a problem such asinflammation. If the voltage is less than 20 kV, electrodes haveresistance therebetween in a space in the atmosphere (in the spacebetween the electrodes), which could impair flow of electrons, therebycausing resins to be not easily electrified. If the voltage exceeds 100kV, sparks or corona discharge could occur between electrodes, whichcould cause burning of the resins.

A distance between the electrodes may be determined, in view of a fiberdiameter of an obtained ultrafine composite fiber, fiber diametervariation, and a property of accumulation of an ultrafine fiber on thecollection-side electrode. For example, the distance between theelectrodes preferably is 2 to 25 cm, and more preferably 5 to 20 cm. Inthe case where the distance is in the above-described range, theelectrification of a resin is caused easily, sparks or corona dischargehardly occur between the electrodes, and therefore there is no problemof burning or the like. If the distance between the electrodes is lessthan 2 cm, sparks or corona discharge tend to occur, possibly burning aresin. If the distance exceeds 25 cm, a resistance between theelectrodes increases, interfering with the flow of electrons, so that aresin tends to be not easily electrified.

A composite-resin-formed product supplied to the supply-side electrodepreferably is supplied in a solid state. When the product passes throughthe supply-side electrode, the composite-resin-formed product may beheated to be in a molten or semi-molten (soften) state. The productpreferably is supplied in a fiber state. When the composite-resin-formedproduct is in a fiber state, a shape of a cross section of the ultrafinecomposite fiber tends to be analogous to a shape of a cross section ofthe fiber-state composite-resin-formed product. Thus, this makes itpossible to control easily a cross-sectional shape of an ultrafinecomposite fiber obtained by electrospinning. The composite-resin-formedproduct (composite fiber) preferably is a monofilament, a multifilamentcomposed of a plurality of monofilaments bundled, or a tow. In theforegoing, the “multifilament” refers to that composed of 2 to 100filaments, and the “tow” refers to that composed of more than 100filaments. Among these, from the viewpoint of electrospinningproperties, the composite-resin-formed product preferably is amultifilament or a tow composed of 2 to 1,000 monofilaments bundled.

A composite-resin-formed product (e.g., a solid composite fiber) isirradiated with a laser beam immediately after the beam passes thesupply-side electrode, whereby the composite-resin-formed product isheated and melted. Even in the case where the composite-resin-formedproduct is molten or semi-molten preliminarily, thecomposite-resin-formed product is made to have a low viscosity byadditional heating and melting in the space between the electrodes,whereby extensibility can be improved. Examples of the laser beaminclude a laser beam emitted from light sources such as a YAG laser, acarbon dioxide gas (CO₂) laser, an argon laser, an excimer laser, and ahelium-cadmium laser. Among these, the laser beam emitted from a carbondioxide gas laser is preferred, from the viewpoint of a high heatabsorptivity with respect to a polymer resin, a high power sourceefficiency, and a high composite fiber melting property. The laser beamhas a wavelength of, for example, 200 nm to 20 μm, preferably 500 nm to18 μm, more preferably 1 to 16 μm (particularly 5 to 15 μm)approximately. As another means for heating and melting thecomposite-resin-formed product, a known means such as a means of nearinfrared radiation in a wavelength range of 780 nm to 2.5 μm may beused.

The method for irradiation with a laser beam is not limitedparticularly, but the method of irradiation with a laser beam in aspotlight form is preferred, since the composite fiber is irradiatedwith a beam locally. A beam diameter of the laser beam irradiating acomposite fiber can be selected according to the shape of the compositefiber. A specific beam diameter may be, in the case of a linear-formresin for example (e.g., monofilament, multifilament, tow, etc), greaterthan an average diameter of the linear-form resin, and is in a range of,for example, 0.5 to 30 mm, preferably 1 to 20 mm, more preferably 2 to15 mm (particularly 3 to 10 mm). The beam diameter may be such that aratio of the beam diameter to the average diameter of the linear-formresin is 1:1 to 100:1, preferably 1:2 to 1:50, and more preferably 1:3to 1:30 (particularly 1:5 to 1:20).

In the case where the composite-resin-formed product is irradiated witha laser beam after the beam passes through the supply-side electrode,whereby the product is heated and melted, a distance between an end on aside of the supply-side electrode from which the composite-resin-formedproduct comes out and a portion of the product irradiated with the laserbeam is 1 to 6 mm preferably. More preferably the distance is 2 to 4 mm.If the distance is less than 1 mm, the laser-irradiated portion is veryclose to the electrode, thereby causing the temperature of the electrodeto rise. As a result, the resin-formed product is heated longer in time,which could cause the resin to be decomposed. If the distance exceeds 6mm, the amount of electric charges in the resin-formed productelectrified when the product passes through the supply-side electrodedecreases, and even if such a portion of the product is heated andmolten by the laser beam, the resin in the molten state is not easilyextended uneasily toward the collection-side electrode.

An output power of the laser beam necessary for melting thecomposite-resin-formed product may be controlled so that the product canbe heated to a temperature that is not lower than a melting point of aresin having the highest melting point among resins composing thecomposite-resin-formed product, and at which none of the resinscomposing the composite-resin-formed product is ignited or decomposed.In short, the composite-resin-formed product may assume a viscous state.The temperature at which the composite-resin-formed product assumes aviscous state varies with the rate of supply of thecomposite-resin-formed product, the output power of the laser beam, thedistance between the laser and the composite-resin-formed product, andthe fineness of the composite-resin-formed product. In the case of alaser beam, the heating temperature preferably is 160° C. to 1200° C.,and more preferably 600° C. to 800° C. If the temperature is lower than160° C., the amount of heat for heating the product is insufficient,whereby melting failures occur. This causes the product to fail toassume a viscous state, making it difficult to form the product into anultrafine fiber. If the temperature is over 1,200° C., the resin mightbe burned or decomposed, which makes it impossible to form the resininto fibers. The specific output power of the laser beam may be selectedappropriately depending on the physical property (melting point), shape,fineness, and supply rate of the composite-resin-formed product used.The output power thereof may be, for example, 3 to 100 mA, preferably 3to 50 mA, and more preferably 6 to 40 mA If the output power of thelaser beam is less than 3 mA, the power is not enough to melt the resin.Irradiation conditions of the laser beam may be controlled by themelting point of the composite-resin-formed product. In the case where,however, the composite-resin-formed product is a filament having a smalldiameter and a high voltage is applied thereto, the control of theconditions is performed by the control of the output power of the laserbeam from the viewpoint of conveniences. One or a plurality of pointsaround the composite-resin-formed product is irradiated with the laserbeam.

The composite-resin-formed product molten is extended toward thecollection-side electrode in accordance with the electric attractionforce. Here, the extension ratio is 100 to 1000 times, preferably 200 to800 times, and more preferably 300 to 500 times. The product is extendedat such a ratio, thereby being formed into ultrafine fibers. Here,hyperextension possibly could occur. As a result, the diameter of theultrafine composite fiber containing a resin having a volume specificresistance exceeding 10¹⁵ Ω·cm can be decreased to 5 μm or less. Underpreferable conditions, the diameter can be decreased to 3 μm or less,and under more preferable conditions, the diameter can be decreased to 1μm or less.

In the present invention, the composite-resin-formed product preferablyis of a sea-island type, a splittable type, or a core-sheath type asviewed in a cross section thereof. When the product has such a crosssection, a resin that easily is electrified upon passing through thesupply-side electrode can be arranged selectively.

It should be noted that the fiber diameter is derived from a diameter ofthe fiber when the fiber has a round cross section. The fiber diameter(diameter) is determined from a fiber cross section or a fiber sideface.

In the case where the fiber has a cross section in an irregular shape(polygonal, elliptic, hollow, “C”-shaped, “Y”-shaped, “X”-shaped, orindefinite-shaped cross section, etc.), a circular shape having the samearea as that of the cross section of the fiber is given and the diameterof the round shape is determined, which is assumed to be the diameter ofthe fiber. Therefore, in the case of an oddly-shaped fiber, the fiberdiameter cannot be derived from the side face of the fiber.

The resin exposed on 30% or more of the surface of the part of thecomposite-resin-formed product that is passed through the supply-sideelectrode preferably has a volume specific resistance of 10⁶ to 10¹⁴Ω·cm. More preferably, the resistivity is 10⁷ to 10¹⁴ Ω·cm. With thisconfiguration, the composite-resin-formed fiber is electrified easilywhen it is passed through the supply-side electrode.

Further, even a resin having a volume specific resistance of more than10¹⁵ Ω·cm could possibly be made suitable for electrospinning, bydecreasing an apparent volume specific resistance using one of thefollowing techniques alone or a plurality of the same in combination bythe time when it is electrospun: processing techniques for decreasing aresistivity of a resin, such as kneading a master batch that causes avolume specific resistance of a resin to decrease (e.g., a master batchcontaining a filler such as carbon or a metal salt), corona processing,fluorine processing, and electret processing; and processing techniquesof coating the composite-resin-formed product with an oil agent thatallows a volume specific resistance to decrease (e.g., an anionicsurfactant, a cationic surfactant, or a nonionic surfactant) orimmersing the composite-resin-formed product with such an oil agent.

It should be noted that the “apparent volume specific resistance” refersto a value obtained by determining a volume specific resistance (ASTMD-257) of a sample obtained by subjecting a resin portion of the sampleto the aforementioned processing technique, the volume specificresistance (ASTM D-257) being determined usually with respect to aresin.

In other words, the volume specific resistance is not a volume specificresistance of a resin itself, but a value indicative of a volumespecific resin of a processed resin.

A configuration in which a resin having a volume specific resistance of10¹⁵ Ω·cm or less is exposed on 30% or more of a surface of acomposite-resin-formed product has the following advantage: even if aresin having a volume specific resistance of more than 10¹⁵ Ω·cm andtherefore becoming electrified uneasily is included in theconfiguration, the resin having a volume specific resistance of 10¹⁵Ω·cm or less is sufficiently electrified and electrospun, and theinfluences of the same cause even the resin having a volume specificresistance of more than 10¹⁵ Ω·cm to be electrospun and extended at thesame time.

It is essential only that a resin having a volume specific resistance of10¹⁵ Ω·cm or less is exposed on 30% or more of a surface of acomposite-resin-formed product, which makes it possible to form anultrafine composite fiber even with a combination of a resin having ahigh volume specific resistance and a resin having a low volume specificresistance, and under preferable conditions. This could make it possibleto obtain an ultrafine composite fiber having a diameter of 3 μm orless.

Needless to say, there is no problem in the combination of resins havingvolume specific resistances of 10¹⁵ Ω·cm or less.

In the present invention, a proportion of a resin having a volumespecific resistance of 10¹⁵ Ω·cm or less is 10 percent by mass (mass %)or more, preferably 30 mass % or more, and more preferably 50 mass % ormore in a composite-resin-formed product. When the proportion of theabove-described resin is in this range, an ultrafine composite fiber canbe obtained stably. When the proportion of the resin is less than 10mass %, even the provision of the resin on the surface of thecomposite-resin-formed product is not effective for achievingultrafineness, since an uneasily-electrified resin having a volumespecific resistance of more than 10¹⁵ Ω·cm, which is in a large part dueto the small total amount of resins in the composite-resin-formedproduct, has to be extended toward the collection-side electrode uponelectrospinning, which makes it difficult to form an ultrafine fiber.

Even in the case where an uneasily electrified resin having a volumespecific resistance of 10¹⁶ Ω·cm or more, such as olefin (e.g.,polypropylene, or polyethylene) is provided, excellent electrospinningcan be performed if there is 10 mass % or more of a resin having avolume specific resistance of 10¹⁵ Ω·cm or less. If a resin having avolume specific resistance of 10¹⁶ Ω·cm or more, such as olefin, and aresin having a volume specific resistance of 10¹⁵ Ω·cm or less are used,a preferable proportion of the resin having a volume specific resistanceof 10¹⁵ Ω·cm or less is 10 mass % to 70 mass %. More preferably, theproportion is 35 mass % to 60 mass %. If the resin having a volumespecific resistance of 10¹⁵ Ω·cm or less accounts for less than 10 mass%, the ultrafineness is not easily achieved, and if the resin accountsfor more than 70 mass %, there is no problem in electrospinning, but itis difficult to stably obtain a composite-resin-formed product as a rawmaterial, since an olefin portion is extremely small when thecomposite-resin-formed product is produced.

Further, it is preferable that the component exposed on 30% or more ofthe surface of the composite-resin-formed product is at least oneselected from ethylene-vinyl alcohol copolymers, polyesters, nylons, andpolyurethanes, and that a component of the other phase is at least oneselected from polyolefins, polyesters, nylons, and polylactic acids.

The component exposed on 30% or more of the surface of thecomposite-resin-formed product particularly preferably is anethylene-vinyl alcohol copolymer, from the viewpoint that anethylene-vinyl alcohol copolymer can be highly electrified and has agreat extensibility upon electrospinning and has excellentbiocompatibility. The ethylene-vinyl alcohol copolymer has a volumespecific resistance of, preferably, 10⁶ to 10⁵ Ω·cm, more preferably 10⁷to 10⁹ Ω·cm, and further more preferably 10⁷⁵ to 10^(8.5) Ω·cm.

The ethylene-vinyl alcohol copolymer is obtained by saponification of anethylene-vinyl acetate copolymer. The content of ethylene in theethylene-vinyl alcohol copolymer is not limited particularly, butpreferably is 25 to 70 percent by mole (mol %), and more preferably 30to 65 mol %. Commercially available examples of the same include “EVAL”(trade name) produced by Kuraray Co., Ltd., “Soarnol” (trade name)produced by Nippon Synthetic Chemical Industry Co., Ltd., and suchcommercially available products can be used in the present invention.The melting point of an ethylene-vinyl alcohol copolymer varies with thecontent of ethylene and vinyl alcohol contained therein, and forexample, an ethylene-vinyl alcohol copolymer containing 38% of ethylenehas a melting point of 171° C., and an ethylene-vinyl alcohol copolymercontaining 55% of ethylene has a melting point of 142° C. Theethylene-vinyl alcohol copolymer used may be an ethylene-vinyl alcoholcopolymer of a single type, or alternatively, a mixture of two or moretypes of ethylene-vinyl alcohol copolymers that are different in thecontent of ethylene.

The ethylene-vinyl alcohol copolymer preferably has a melting point of100° C. to 190° C., more preferably 120° C. to 180° C., and furtherpreferably 140° C. to 175° C. When the melting point of ethylene-vinylalcohol copolymer is 100° C. or higher, the composite-resin-formedproduct is easily formed into fibers, and when the melting point ofethylene-vinyl alcohol copolymer is 190° C. or lower, the thermalbonding of intersection points of fibers can be performed at arelatively low temperature.

The component of the other phase is not limited particularly, and, forexample, one of the following, or a mixture of two or more of thefollowing, may be used as the component: polyolefins such aspolyethylene, polypropylene, polybutene, polymethylpentene,polytrimethylene terephthalate, ethylene-propylene copolymer; polyesterssuch as polyethylene terephthalate, and polybutylene terephthalate;polyamides such as nylon 6, and nylon 66; and polystyrene. The componentof the other phase preferably has a melting point in a range of 150° C.to 300° C., from the viewpoint of obtaining a fiber strength. It shouldbe noted that examples of a polymer having a melting point in a range of150° C. to 300° C. include polypropylene (160° C. to 175° C.),polymethyl pentene (230° C. to 240° C.), polyethylene terephthalate(212° C. to 265° C.), polybutylene terephthalate (220° C. to 267° C.),nylon 6 (210° C. to 220° C.), and nylon 66 (255° C. to 265° C.)(“Plastic Data Book”, edited by Asahi Kasei Amidas Corporation, PlasticsEditorial Section, published on Dec. 1, 1999 by Kogyo ChosakaiPublishing Inc., pages 7 to 11).

Particularly a composite fiber such as a core-sheath-type compositefiber, a sea-island-type composite fiber, or a splittable-type compositefiber in which a component that is exposed on 30% or more of a surfaceis an ethylene-vinyl alcohol copolymer and a composite of the otherphase is a polylactic acid can be used in the following materials to beembedded in a living body, since both components are highlybiocompatible and such a composite fiber does not contain a solvent inthe fiber. The exemplary materials in which the foregoing compositefiber can be used include materials to be embedded in a living body suchas surgical sutures, stents, and artificial joints; medical materialssuch as hematostatic materials, cell culture substrates, masks, and bodyfluid absorbing pads; and cosmetics such as face masks, andinterpersonal wipes.

The ultrafine composite fibers preferably do not contain solvents in thefiber itself. More preferably, the resin composing the ultrafinecomposite fiber does not contain an organic solvent or a componentoriginating from an organic solvent. In the case where the ultrafinecomposite fiber does not contain a solvent in the fiber itself, it canbe applied to a field of products brought into contact with humans and amedical field, since a solvent or a component originating from such asolvent, which are generally toxic with respect to humans and animals,never is eluted from such a fiber. It should be noted that the“component originating from an organic solvent” refers to a componentsuch as a solvent chemically changed by heat or electric charges.

In order to obtain a configuration in which the ultrafine compositefiber does not contain any solvent in the fiber itself, thecomposite-resin-formed product in a solid state may contain no solvent.The composite-resin-formed product not containing a solvent is obtainedby, for example, spinning a resin by a conventional melt-spinningmethod, in the case where the composite-resin-formed product is in afiber form.

The “solvent” refers to that which can dissolve or disperse a resintherein and exhibits toxicity with respect to humans and animals.Examples of the solvent include hexane, benzene, toluene, diethyl ether,chloroform, ethyl acetate, tetrahydrofuran, methylene chloride, acetone,acetonitrile, dimethylformamide, dimethyl sulfoxide, acetic acid,1-butanol, 1-propanol, 2-propanol, methanol, ethanol, and formic acid.As described above, the ultrafine composite fiber preferably does notcontain a solvent in the fiber itself.

The ultrafine composite fiber preferably has a diameter of 20 μm orless, more preferably a diameter in a range of 0.2 to 17 μm, andparticularly preferably a diameter in a range of 0.5 to 5 μm. Theultrafine fiber having a diameter in such a range is difficult to obtainby a normal melt-spinning technique.

A composite fiber obtained by electrospinning preferably has phaseseparation of two components, that is, one component that is exposed on30% of or more a surface of the fiber, and another component having theother phase. The phase separation causes the components to easilyseparate from each other, whereby an ultrafine fiber can be obtainedeasily.

The ultrafine composite fiber preferably is of a sea-island type, asplittable type, or a core-sheath type as viewed in its cross-section.In the case where the composite-resin-formed product is a multifilamentor a tow, an ultrafine fiber obtained has such a cross-sectional shapethat the multifilament or the two becomes one fiber in some cases. Anexemplary case is as follows: in the case where a tow composed of 600core-sheath-type composite fibers bundled is used as acomposite-resin-formed product, an ultrafine composite fiber obtained byextension through electrospinning is of a sea-island type in which thenumber of segments of an island component is 1 to 600 apparently. Anultrafine fiber having such a cross-sectional shape is also included inexamples of the sea-island type and/or core-sheath type composite fiberin the present invention.

The composite fiber preferably contains 30 to 90% of the componentexposed on 30% or more of the surface, and 70 to 10% of the component ofthe other phase. More preferably, the ratio of the former fiber to thelatter fiber is 35:65 to 60:40. When the ratio is in the foregoingrange, the product is easily electrified in the space between theelectrodes, whereby excellent spinning properties are achieved.

It also is possible to remove any of the components that compose theultrafine composite fiber obtained in the present invention, so as toobtain an ultrafine fiber composed of the remaining components. Thismakes it possible to obtain a further finer fiber. Still further, thisalso makes it possible to obtain an ultrafine fiber containing only aspecific polymer that is intended to remain.

In the sea-island structure or the splittable structure that can besplit into a multiplicity of components, the segment unit of eachindividual resin component is small (as viewed in a cross-section, thestructure is a group of very small units). Therefore, it is consideredthat the structure is influenced easily by the electrification of theresins at the supply-side electrode, and by the heating and melting ofthe resins by a laser beam, and therefore, which allows the entiresegments to be homogeneously influenced.

A finer fiber may be taken out from the obtained ultrafine compositefiber by further removing either one of the resin components. Theremoving method is not limited particularly, and a known method using anacid, an alkali, an organic solvent, or the like may be used. Theremoving method may be selected appropriately, according to a solubilityproduct of a resin with respect to a solvent.

Particularly in the case of a core-sheath-type composite fiber in whichthe sheath and the core are formed with an ethylene-vinyl alcoholcopolymer and a polylactic acid, respectively, water at 80° C. to 100°C., or an alkaline aqueous solution obtained by dissolving potassiumhydroxide, sodium hydroxide, or sodium hydrogencarbonate in water may beused, in order to remove the polylactic acid. In order to remove theethylene-vinyl alcohol copolymer, dimethyl sulfoxide (DMSO) at normaltemperature (20° C. to 30° C.), isopropyl alcohol heated to 60° C.,2-propanol or the like may be used.

The extended ultrafine composite fiber is allowed to have various shapesdepending on the method for heating the composite-resin-formed productas a raw material. For example, homogeneous heating allows a fiberhaving a cross-section in a shape analogous to that of thecomposite-resin-formed product to be obtained. Heating thecomposite-resin-formed product from one side causes bias in a moltenstate as viewed in the cross-section, thereby allowing a fiber having anunsymmetrical cross-section shape to be obtained. The reason for this isconsidered as follows: one resin side face is molten sufficientlythereby being extended, while an opposite side face has a greatermelting viscosity as compared with the other side and is not extendedsufficiently, which results in that an ultrafine composite fiberobtained has a shape different from that of the composite-resin-formedproduct. Specifically, the circular cross-section of thecomposite-resin-formed product is turned into a C-letter-shapedcross-section of the obtained ultrafine composite fiber, or the corecomponent is split into two or more, whereby a further finer fiber couldbe obtained.

The ultrafine composite fibers are accumulated on the collection-sideelectrode, whereby a fiber structure can be obtained. The fiberstructure in its accumulated state may be collected directly from thecollection electrode. Alternatively, the collection-side electrode maybe formed in a conveyer form, and the position at which fibers areaccumulated may be shifted continuously so that a sheet-form fiberstructure can be produced continuously. Further alternatively, anothermethod of collecting a fiber structure is as follows: a metal mesh, awoven fabric, a nonwoven fabric, or a sheet of paper is arranged on thecollection-side electrode, and ultrafine composite fibers areaccumulated on such a sheet-form material, whereby a laminate-like fiberstructure can be obtained. Further, fibers may be accumulated in anon-sheet shape, such as an article having a certain thickness such as acartridge-type filter. In the present invention, the “fiber structure”refers to a fiber accumulation such as an article having a certainthickness, for example, a fiber sheet, a filter, and the like.

A physical object to be accumulated preferably is grounded so as to haveno potential difference from the collection-side electrode. If noproblem arises in the production, however, they do not have to begrounded intentionally, and may be held slightly afloat on thecollection-side electrode.

In the present invention, a combination of EVOH and PP also is apreferable example. From an ultrafine composite fiber obtained, theethylene-vinyl alcohol copolymer (EVOH) may be removed further, so thata polypropylene (PP) fiber alone can be obtained. The removal ofethylene-vinyl alcohol copolymer (EVOH) is achieved by, for example,immersing the obtained fiber in an aqueous solution containing2-propanol, or dimethyl sulfoxide (DMSO). The polypropylene ultrafinefiber thus obtained as a result of the removing of EVOH possibly couldbe further finer than the foregoing ultrafine composite fiber. Thepolypropylene ultrafine fiber preferably has a fiber diameter of 13 μmor less, and more preferably, 5 μm or less. For example, an ultrafinecomposite fiber formed to have a diameter of about 1.2 μm and acore-sheath ratio of 50:50 under preferable conditions may be subjectedto removal of ethylene-vinyl alcohol copolymer, whereby a further finerpolypropylene ultrafine fiber having a diameter of 0.3 μm can beobtained.

The extended ultrafine composite fiber and/or polypropylene as a corecomponent is allowed to have various shapes depending on the method forheating the composite-resin-formed product as a raw material. Forexample, homogeneous heating allows a fiber having a cross-section in ashape analogous to that of the composite-resin-formed product to beobtained. Heating the composite-resin-formed product from one sidecauses bias in a molten state as viewed in the cross-section, therebyallowing a fiber having an unanalogous cross-section shape to beobtained. The reason for this is considered as follows: one resin sideface is sufficiently melted thereby being extended, while an oppositeside face has a greater melting viscosity as compared with the otherside, which results in that an ultrafine composite fiber has a shapedifferent from that of the composite-resin-formed product. Specifically,the circular cross-section of the composite-resin-formed product isturned into a C-letter-shaped cross-section of the obtained ultrafinecomposite fiber, or the core component is split into two or more,whereby a further finer fiber could be obtained.

The following will describe a manufacturing method, referring to thedrawings. FIG. 1 specifically illustrates an electrospinning deviceaccording to an example of the present invention. In thiselectrospinning device 11 a voltage generator 3 applies a voltage acrossthe supply-side electrode 1 and the collection-side electrode 2, and alaser irradiation equipment 4 emits a laser beam in a directionindicated by an arrow X toward a position immediately under thesupply-side electrode 1. A distance between the supply-side electrodeand the collection-side electrode preferably is 2 to 25 cm. The distanceis more preferably 5 to 20 cm. If the distance between the electrodes issmaller than 2 cm, sparks or corona discharge occur due to theapplication of a high voltage. If the distance is greater than 25 cm,the effect of electric attraction force decreases, and there is apossibility that a molten fiber is not extended toward thecollection-side electrode. A composite fiber 7 as a raw material istaken out of housed fibers 6 contained in a container 5, passes viaguides 8 and 9, and a supply roller 10, and thereafter is supplied tothe electrospinning device 11. The composite fiber as a raw material maybe supplied from a bobbin around which the material is wound. Thecomposite fiber 7 is electrified when passing the supply-side electrode.The composite fiber 7, in this electrified state, is irradiated with thelaser beam emitted from the laser irradiation equipment 4 in the arrow Xdirection, so as to be heated and melted, immediately under thesupply-side electrode 1, and is extended toward the collection-sideelectrode by the electric attraction force. Here, the composite fiber isextended in a direction indicated by an arrow Y, becoming ultrafine.“12” denotes a fiber structure (sheet) obtained from accumulation of thecomposite fibers that have been made ultrafine.

In FIG. 1, in the case where a plurality of composite fibers as rawmaterials are used, for example, six composite fibers 7 a to 7 f asshown in FIG. 11, a position immediately under the supply-side electrode1 may be irradiated by the laser irradiation equipment 4 via areflection plate mirror 13. Here, by operating the reflection platemirror 13, the laser beam is swung by an angle θ between a directionindicated by an arrow X1 and a direction indicated by an arrow X2. Thisallows all of the composite fibers 7 a to 7 f as raw materials to beirradiated with a laser beam. This makes spinning possible even if aplurality of composite fibers or a nonwoven fabric is used as thecomposite-resin-formed product.

FIG. 2 schematically illustrates another electrospinning deviceaccording to another example of the present invention. Thiselectrospinning device 20 applies a voltage from a high-voltage terminal22 to a supply-side electrode 21 attached to a polyimide resin plate 23.The supply-side electrode preferably is in a needle shape. In such aneedle electrode, the needle length preferably is 5 to 30 mm, and morepreferably 10 to 20 mm. If the needle length is less than 5 mm, thedirection in which the composite fiber as a raw material is to beextruded is not set, which makes it difficult to guide the compositefiber to the laser beam irradiation position. If the needle length ismore than 30 mm, a resistance is applied to the raw material fiber whenit passes through the needle, and the raw material fiber cannot beextruded smoothly. The needle preferably has an inner diameter of 10 to2000 μm, and more preferably 20 to 1650 μm. If the inner diameter isless than 20 μm, the number of fibers to be processed decreases, andalso it is difficult to pass the raw material fibers therethrough sinceit is narrow. If the inner diameter exceeds 2000 μm, there could be adifficulty in electrifying the fibers sufficiently, particularly inwardparts thereof. The needle electrode does not have to be one in number.In the case where a large amount of a material is to be electrospun atone time, a bundle of a plurality of thin needles, rather than one thickneedle, is advantageous regarding the guiding property for guiding rawmaterial fibers toward the laser beam irradiation position. The numberof needles preferably is 1 to 1,000, and more preferably 1 to 300. Thecollection-side electrode 24 is earthed. A position immediately underthe supply-side electrode 21 is irradiated with laser beams indirections indicated by arrows X1 and X2 from a plurality of laserirradiation equipment 25. Composite fibers 7 as a raw material areelectrified when passing the supply-side electrode 21. When thecomposite fibers 7 in this electrified state are irradiated, at aposition immediately under the supply-side electrode 21, with laserbeams emitted by the laser irradiation equipment 25 in the directionsindicated by arrows X1 and X2, the composite fibers 7 are heated andmolten, and extended toward the collection-side electrode 24 by anelectric attraction force. Here, the composite fibers 7 are extendedseveral hundred times in length, for example, in a direction indicatedby the arrow Y, thereby being made ultrafine. “29” denotes anaccumulation of composite fibers that have been made ultrafine. In aheating-extending region 28, if the temperature decreases with theincreasing proximity from the laser beam irradiation position to thecollection-side electrode, the resin starts crystallizing while thecomposite fibers are being extended, which makes it difficult to drawthe resin to make finer fibers. Therefore, the heating-extending region28 preferably is heated by transmitting heat from a heating means suchas a heater or an oil tank, so that the fibers are not cooled rapidly.The temperature of the heating-extending region depends on the type ofthe fiber, but the region may be heated to a temperature in a range froma glass-transition point to a melting point of a synthetic fiber.Specifically, the temperature in the heating-extending region preferablyis 50 to 300° C., and more preferably 100 to 200° C. The heatingpreferably is performed by a method using electricity, since minutetemperature adjustment is performed easily by such a method

FIG. 12A is a cross-sectional view of a composite fiber as acomposite-resin-formed product as a raw material used in one example ofthe present invention. This composite fiber 70 is a monofilament thathas a core-sheath structure having a core 71 made of polypropylene (PP)and a sheath 72 made of an ethylene-vinyl alcohol polymer (EVOH). FIG.12B is a cross-sectional view of an ultrafine composite fiber obtainedin one example of the present invention. This composite fiber 73 has acore-sheath structure having a polypropylene (PP)-made core 74 that ismade ultrafine, and an ethylene-vinyl alcohol copolymer (EVOH)-madesheath 75 that is made ultrafine.

FIG. 13A is a cross-sectional view of a composite fiber that is acomposite-resin-formed product as a raw material used in another exampleof the present invention. In this example, a composite fiber 76 is amultifilament obtained by temporarily fixing a plurality ofcore-sheath-type filaments into a bundle, each filament having a core 77made of polypropylene (PP) and a sheath 78 made of an ethylene-vinylalcohol copolymer (EVOH). The filaments can be fixed temporarily to bebundled, for example, by pouring boiled water over a plurality ofcomposite fibers. FIG. 13B is a cross-sectional view of an ultrafinecomposite fiber obtained in another example of the present invention.The composite fiber 80 has a core-sheath structure having polypropylene(PP)-made cores 81 that are made ultrafine and ethylene-vinyl alcoholcopolymer (EVOH)-made sheaths 82 that are made ultrafine.

EXAMPLE

The following describes the present invention in more detail, whilereferring to Examples. It should be noted that the present invention isnot limited to Examples shown below.

1. Raw Material Resin

The following resins are used:

(1) Polyethylene terephthalate (PET): “T200E” (trade name) produced byToray Industries, Inc. having a melting point of 255° C. and anintrinsic viscosity (IV) of 0.64.

(2) Polypropylene (PP): “SA03” (trade name) produced by JapanPolypropylene Corporation, having a melting point of 161° C., and a meltflow rate of 30 determined according to JIS-K-7210 (MFR; temperatureupon determination: 230° C., load: 21.18 N (2.16 kgf))

(3) Nylon 6 (NO: “1015B” (trade name) produced by Ube Industries, Ltd.,having a molecular weight of 15000

(4) High-density polyethylene (PE): “HE481” (grade name) produced byJapan Polyethylene Corporation, having a melting point of 130° C., and amelt flow rate of 12 determined according to JIS-K-7210 (MFR;temperature upon determination: 190° C., load: 21.18 N (2.16 kgf))

(5) Ethylene-vinyl alcohol copolymer (EVOH) used in Experiment No. 10:“K3835BN” produced by Nippon Synthetic Chemical. Industry Co., having amelting point of 171° C., and a melt flow rate of 35 determinedaccording to JIS-K-7210 (MFR; temperature upon determination: 230° C.,load: 21.18 N (2.16 kgf)).

(6) Ethylene-vinyl alcohol copolymer (EVOH) used in Experiments Nos. 11and 12: “SG544” produced by Nippon Synthetic Chemical. Industry Co.,having a melting point of 170° C. and a melt flow rate of 45 determinedaccording to JIS-K-7210 (MFR; temperature upon determination: 230° C.,load: 21.18N (2.16 kgf)).

(7) Polylactic acid (PLA) used in Experiments Nos. 11 and 12: “U'z S-32”produced by Toyota Motor Corp., having a melting point of 179° C. and avolume specific resistance of 9×10¹⁶ (Ω·cm).

2. Production of Raw-Material Composite-Resin-Formed Product

A raw-material composite-resin-formed product was obtained as follows:an unextended fiber was obtained by conventional melt-spinning, and wasused as a composite-resin-formed product (composite fiber) as a rawmaterial.

3. Electrospinning

As an electrospinning device, the device shown in FIG. 2 was used underthe conditions as shown in Table 2.

Laser device: PIN-30R manufactured by Onizca Glass Co., Ltd. (ratedoutput: 30 W, wavelength: 10.6 μm, beam diameter: 6 mm)

Voltage applied across electrodes: 35 kV

Distance between electrodes: 8 cm

Raw material fiber feeding rate: 6.0 mm/min

Ambient temperature: 28° C.

Laser intensity: 100 V, 8 mA

Distance between supply-side electrode and laser irradiation position: 4mm

Supply-side electrode: UN series, 20Gx 15 (as one unit), manufactured byUnicontrols Co., Ltd.

4. Method for Measuring Fiber Diameter

Side faces of fibers were observed using a scanning electron microscope(SEM, “S-3500” (trade name) manufactured by Hitachi, Ltd.,magnification: 1500 times), and an average was determined based onmeasurement results of arbitrarily selected 30 fibers.

Example 1

A first component, a second component, a cross-sectional structure, aresin ratio, and a fiber diameter of a raw-materialcomposite-resin-formed product of each of Experiments Nos. 1 to 12 areas shown in Table 2.

Ultrafine fibers were produced by the electrospinning method shown inFIG. 2, with use of these raw-material composite-resin-formed products.The electrospinning conditions and the fiber diameters of the obtainedultrafine fibers are shown collectively in Table 2.

TABLE 2 Spinning condition Raw-material fiber Electric Distance Cross-Fiber field between Laser Experiment Core Sheath sectional Resindiameter intensity Electrodes intensity No. component componentstructure ratio (μm) (kV) (cm) (mA) 1 PET Ny Core-sheath 50/50 360 32.520 20 2 Ny PET Core-sheath 30/70 410 32.5 20 40 3 Ny PET Core-sheath50/50 410 32.5 20 40 4 Ny PET Core-sheath 70/30 410 32.5 20 40 5 PET PE16-part split 50/50 120 32.5 20 40 6 PET PE 16-part split 50/50 220 32.55 16 7 PET PE Core-sheath 50/50 420 32.5 5 20 (Comp. Ex.) 8 PE PETCore-sheath 50/50 420 32.5 5 20 9 PP PE Core-sheath 50/50 220 32.5 20 10(Comp. Ex.) 10  PP EVOH Sea-island 50/50 360 32.5 20 20 37 islands 11 PLA EVOH 16-part split 50/50 350 32.5 20 20 12  PLA EVOH Core-sheath50/50 400 32.5 20 20 Spinning condition Fiber Heating-extending diameterSpinning region after Fineness after Experiment rate temperatureelectrospinning Removed Removal Ratio removal No. (mm/min) ° C.) (μm)Component (%) (μm) 1 5.41 160 4.70 — — — 2 5.41 160 — — — — 3 5.41 1600.56 — — — 4 5.41 160 — — — — 5 20.3 160 1.93 — — — 6 5.41 160 1.08 — —— 7 5.41 160 Unspinnable — — — (Comp. Ex.) 8 5.41 160 2.15 — — — 9 5.41160 Unspinnable — — — (Comp. Ex.) 10  5.41 160 2.21 — — — 11  5.41 None1.21 PLA 100 0.78 (Normal Temp.) 12  5.41 None 1.85 EVOH 100 1.12(Normal Temp.)

Solvent removal conditions of Experiments Nos. 11 and 12 in Table 2 areas follows:

A Removal of EVOH

1 g of a composite fiber obtained by electrospinning was placed in 20 mlof dimethyl sulfoxide, and was stirred at normal temperature (20 to 30°C.) for 15 hours. Thereafter, suction filtration of a solvent andsubstitution cleaning with methanol were performed, followed by dryingat normal temperature for one hour, whereby a PLA fiber was obtained.

B. Removal of PLA

1 g of a composite fiber obtained by electrospinning was placed in 200ml of 5 mass % aqueous solution of potassium hydroxide, and was stirredat 80° C. for 15 hours. Thereafter, suction filtration of a solvent andsubstitution cleaning with pure water were performed, followed by dryingat 60° C. for one hour, whereby an EVOH fiber was obtained.

As is clear from Table 2, in each of Experiments Nos. 1 to 6, 8, and 10to 12, a part of the composite-resin-formed product that passed throughthe supply-side electrode had a resin having a volume specificresistance of 10¹⁵ Ω·cm or less on 50% or 100% of a surface of theforegoing part, which resulted in that excellent spinning propertieswere achieved, and ultrafine fibers were obtained. No solvent waspresent in the obtained composite fibers.

In contrast, in each of Experiments Nos. 7 and 9, the sheath componentof the composite fiber was polyolefin, and therefore, the resin having avolume specific resistance of 10¹⁵ Ω·cm or more and not easilyelectrified was present on 100% of the surface of the composite fiber.As a result, it was impossible to perform spinning.

A scanning electron photomicrograph (SEM, magnification: 20,000 times)of a cross section of an ultrafine fiber obtained in Experiment No. 3 isshown in FIG. 3, and a specific cross-sectional view of the fiber isshown in FIG. 4. In FIG. 4, “30” denotes an ultrafine composite fiber ofa core-sheath structure, “31” denotes Ny, and “32” denotes PET. Since itwas a core-sheath type composite fiber in which PET having a volumespecific resistance of 10¹⁵ Ω·cm or less was the sheath component andhence was exposed on 100% of a surface of the fiber, excellent spinningproperties were obtained, and the extended fiber was in a phaseseparation state. In the present case under these conditions, since thecomposite-resin-formed product having a core-sheath structure was usedas a raw material, an ultrafine composite fiber having an analogousshape was obtained.

A scanning electron photomicrograph (SEM, magnification: 10,000 times)of a cross section of an ultrafine fiber obtained in Experiment No. 5was shown in FIG. 5, and a schematic cross-sectional view of the fiberis shown in FIG. 6. In FIG. 6, “40” denotes an ultrafine composite fiberhaving a 16-part-split structure, “41” denotes PET, and “42” denotes PE.Since the fiber was a splittable-type composite fiber in which Ny havinga volume specific resistance of 10¹⁵ Ω·cm or less was used and the resinhaving a low volume specific resistance was exposed on 50% of thesurface, excellent spinning properties were exhibited, and the extendedfiber was in a phase separation state. In the present case, since thesplittable-type composite-resin-formed product was used as a rawmaterial, a splittable-type ultrafine composite fiber having ananalogous shape was obtained.

A scanning electron photomicrograph (SEM, magnification: 4,000 times) ofa cross section of an ultrafine fiber obtained in Experiment No. 6 ofthe present invention was shown in FIG. 7, and a cross-sectional view ofthe fiber is shown in FIG. 8. In FIG. 8, “50” denotes an ultrafinecomposite fiber having a 16-part-split structure, “51” denotes PET, and“52” denotes PE. Since the fiber was a splittable-type composite fiberin which Ny having a volume specific resistance of 10¹⁵ Ω·cm or less wasused and the resin having a low volume specific resistance was exposedon 50% of the surface, excellent spinning properties were exhibited, andthe extended fiber was in a phase separation state.

A scanning electron photomicrograph (SEM, magnification: 10,000 times)of a cross section of an ultrafine fiber obtained in Experiment No. 10was shown in FIG. 9, and a schematic cross-sectional view of the fiberis shown in FIG. 10. In FIG. 10, “60” denotes an ultrafine compositefiber having a sea-island structure, “61” denotes PP, and “62” denotesEVOH. It is a sea-island-type composite fiber in which EVOH having avolume specific resistance of 10¹⁵ Ω·cm or less was used and the resinhaving a low volume specific resistance was exposed on 100% of thesurface, excellent spinning properties were exhibited, and the extendedfiber was in a phase separation state. In the present case under theseconditions, since a sea-island-type composite-resin-formed product wasused as a raw material, a sea-island-type ultrafine composite fiberhaving an analogous shape was obtained.

The ultrafine fiber obtained in Experiment No. 11 was substantiallyidentical to those of FIGS. 5 and 6. In FIG. 6, “40” denotes anultrafine composite fiber having a 16-part-split structure, “41” denotesPLA, and “42” denotes EVOH. Since the fiber was a splittable-typecomposite fiber in which EVOH having a volume specific resistance of10¹⁵ Ω·cm or less was used and the resin having a low volume specificresistance was exposed on 50% of the surface, excellent spinningproperties were exhibited, and the extended fiber was in a phaseseparation state. In the present case under these conditions, since thesplittable-type composite-resin-formed product was used as a rawmaterial, a splittable-type ultrafine composite fiber having ananalogous shape was obtained. In Experiment No. 11, PLA was removed,whereby an ultrafine fiber composed of a single component of EVOH wasobtained.

The ultrafine fiber obtained in Experiment No. 12 had a cross sectionsubstantially identical to those of FIGS. 3 and 4. In FIG. 4, “30”denotes an ultrafine composite fiber having a core-sheath structure,“31” denotes PLA, and “32” denotes EVOH. Since the fiber was acore-sheath-type composite fiber in which the sheath component was EVOHhaving a volume specific resistance of 10¹⁵ Ω·cm or less and was exposedon 100% of the surface, excellent spinning properties were exhibited,and the extended fiber was in a phase separation state. In the presentcase under these conditions, since the core-sheath-typecomposite-resin-formed product was used as a raw material, an ultrafinecomposite fiber having an analogous shape was obtained. In ExperimentNo. 12, EVOH was removed, whereby an ultrafine fiber composed of asingle component of PLA was obtained.

Example 2

In Experiments Nos. 13 to 15, fibers having a cross-sectional structureshown in FIG. 12A were used as raw-material fibers that werecomposite-resin-formed products. Cross-sectional structures ofExperiments Nos. 16 and 17 (Comparative Examples) are shown in Table 3.Additionally, core and sheath components, resin ratios, and fiberdiameters of Experiments Nos. 13 to 17 are shown in Table 3.

Ultrafine fibers were produced by the above-described electrospinningmethod, with the use of these raw-material fibers. A scanning electronphotomicrograph (SEM, magnification: 2,000 times) of an ultrafine fiberobtained in Experiment No. 13 is shown in FIG. 14. The fibers obtainedin Experiments Nos. 14 and 15 also had similar appearances.

The ultrafine fibers obtained were immersed sufficiently in a mixedsolution at 90° C. composed of 70 g of 2-propanol and 30 g of distilledwater until ethylene-vinyl alcohol copolymer (EVOH) was dissolvedcompletely therein, so that the ethylene-vinyl alcohol copolymer (EVOH)was removed from the fibers. The scanning electron photomicrographs(SEM, magnification: 2,000 times) of the ultrafine fibers of ExperimentsNos. 1 to 3 after EVOH was removed are shown in FIGS. 15 to 17. Theobtained results are shown in Table 3.

TABLE 3 Fiber diameter after Fiber diameter after removal of sheathRaw-material fiber electrospinning (μm) component (μm) Experiment CoreSheath Cross-sectional Fiber diameter Standard deviation Standarddeviation No. component component structure Resin ratio μm shown in ( )shown in ( ) 13 PP EVOH Core-sheath 30/70 452 1.85 (1.60) 14 PP EVOHCore-sheath 50/50 438 3.46 (0.76) 15 PP EVOH Core-sheath 70/30 472 16.34(3.32)  12.74 (4.59) 16 PP — Homogeneous — 600 Unspinnable — (Comp. Ex.)17 EVOH PP Core-sheath 50/50 450 Unspinnable — (Comp. Ex.)

As is clear from Table 3, in Experiments Nos. 13 to 15, composite fiberseach of which had a core of polypropylene (PP) and a sheath ofethylene-vinyl alcohol copolymer (EVOH) were used as raw materials, andas a result, fiber diameters of 1.85 μm to 16.34 μm after spinning wereachieved. As the proportion of ethylene-vinyl alcohol copolymer (EVOH)was greater, better spinning properties were achieved and the obtainedfibers were finer, but the fiber diameter variation was significant.

Further, after ethylene-vinyl alcohol copolymer (EVOH) was removed, anultrafine fiber made of polypropylene (PP) alone was obtained.

Example 3

In Experiments Nos. 18 to 20, fibers having a cross-sectional structureshown in FIG. 13A were used as raw-material fibers that arecomposite-resin-formed products. Core and sheath components, resinratios, and fiber diameters in these experiments are shown in Table 4.60 of these fibers were bundled. The bundling was performed by fixingends on one side of these 60 fibers with use of a dip, aligning thefibers by slightly pulling the other ends so as to make them tense,temporarily fixing them by pouring boiled water, and drying the same.For example, in Experiment No. 18, 60 filament fibers having a fiberdiameter of 120 μm were bundled, as shown in FIG. 13A

Using these raw-material fibers, ultrafine fibers were produced by theabove-described electrospinning method. A scanning electronphotomicrograph (SEM, magnification: 5,000 times) of an ultrafine fiberobtained in Experiment No. 18 is shown in FIG. 18. The fibers obtainedin Experiments Nos. 19 and 20 also had similar appearances.

The ultrafine fibers obtained were immersed sufficiently in a mixedsolution at 90° C. composed of 70 g of 2-propanol and 30 g of distilledwater until ethylene-vinyl alcohol copolymer (EVOH) was dissolvedcompletely therein, so that the ethylene-vinyl alcohol copolymer (EVOH)was removed from the fibers. The scanning electron photomicrographs(SEM, magnification: 5,000 times) of the ultrafine fibers of ExperimentsNos. 18 to 20 after EVOH was removed are shown in FIGS. 19 to 21. Theobtained results are shown in Table 4.

TABLE 4 Fiber diameter after Fiber diameter after removal of sheathRaw-material fiber electrospinning (μm) component (μm) Core SheathCross-sectional Resin Fiber diameter Standard deviation shown Standarddeviation shown Exp. No. component component structure ratio μm in ( )in ( ) 18 PP EVOH Core-sheath 30/70 120 1.53 (0.55) 0.43 (0.19) 19 PPEVOH Core-sheath 50/50 121 1.21 (0.30) 0.70 (0.09) 20 PP EVOHCore-sheath 70/30 115 1.38 (0.31) 1.28 (0.42)

As is clear from Table 4, in Experiments Nos. 18 to 20, composite fiberseach of which had a core of polypropylene (PP) and a sheath ofethylene-vinyl alcohol copolymer (EVOH) were used as raw materials, andas a result, fiber diameters of 1.21 μm to 1.53 μm after spinning wereachieved. Spinning properties were excellent, ultrafineness was achievedmore stably even as compared with Example 3, and the variation of thefiber diameter variation was small as well. Ultrafine composite fibershaving analogous shapes to those of core-sheath composite fibers as rawmaterials were obtained.

Further, polypropylene (PP) fibers obtained after removal ofethylene-vinyl alcohol copolymer (EVOH) had fiber diameters of 0.43 to1.28 μm.

INDUSTRIAL APPLICABILITY

Ultrafine composite fibers and fiber structures obtained in the presentinvention are useful in filters, battery separators (separators forlithium ion batteries in particular), paper, nonwoven fabrics, skinlayers for synthetic leather substitute, and the like. Further, suchfibers also are useful in materials to be embedded in a living body suchas surgical sutures, stents, and artificial joints; medical materialssuch as hematostatic materials, cell culture substrates, masks, andbodily fluid absorbing pads; and cosmetics such as face masks, andinterpersonal wipes.

1. An ultrafine composite fiber obtained by heating and melting acomposite-resin-formed product in front of a supply-side electrodeand/or in a space between the supply-side electrode and acollection-side electrode and extending the composite-resin-formedproduct by electrospinning, wherein the composite-resin-formed productis a solid-state composite-resin-formed product having two or morephases and including a resin that has a volume specific resistance of10¹⁵ Ω·cm or less and that is exposed on 30% or more of a surface of thecomposite-resin-formed product, and fiber components composing theultrafine composite fiber are in a phase separation state.
 2. Theultrafine composite fiber according to claim 1, containing no solventtherein.
 3. The ultrafine composite fiber according to claim 1, whereinthe composite-resin-formed product is a fiber having a phase structureof a sea-island type, a splittable type, or a core-sheath type.
 4. Theultrafine composite fiber according to claim 1, wherein thecomposite-resin-formed product is a monofilament, a multifilamentcomposed of a plurality of monofilaments bundled, or a tow.
 5. Theultrafine composite fiber according to claim 1, wherein the resincomponent exposed on 30% or more of the surface of thecomposite-resin-formed product is at least one selected fromethylene-vinyl alcohol copolymers, polyesters, nylons, andpolyurethanes, and a resin component of another phase is at least oneselected from polyolefins, polyesters, nylons, and polylactic acids. 6.An ultrafine fiber obtained by removing any of the components composingthe ultrafine composite fiber according to claim
 1. 7. A method formanufacturing an ultrafine composite fiber, the method comprising thesteps of: supplying a composite-resin-formed product to a supply-sideelectrode, wherein the composite-resin-formed product is a solid-statecomposite-resin-formed product having two or more phases and including aresin that has a volume specific resistance of 10¹⁵ Ω·cm or less andthat is exposed on 30% or more of a surface of thecomposite-resin-formed product; heating and melting thecomposite-resin-formed product in front of the supply-side electrodeand/or in a space between the supply-side electrode and acollection-side electrode; and extending the moltencomposite-resin-formed product by electrospinning.
 8. The method formanufacturing an ultrafine composite fiber according to claim 7, whereina heating-extending region is provided between the collection electrodeand a position where the composite-resin-formed product is heated andmolten.
 9. A fiber structure obtained by heating and melting acomposite-resin-formed product in front of a supply-side electrodeand/or in a space between the supply-side electrode and acollection-side electrode, and extending the composite-resin-formedproduct by electrospinning, wherein the composite-resin-formed productis a solid-state composite-resin-formed product having two or morephases and including a resin that has a volume specific resistance of10¹⁵ Ω·cm or less and that is exposed on 30% or more of a surface of thecomposite-resin-formed product, and fiber components composing theultrafine composite fiber are in a phase separation state.